Electrodeless fluorescent lamps have recently been introduced into the market for indoor, outdoor, industrial and commercial applications. An advantage of electrodeless lamps is the absence of internal electrodes and heating filaments which are life-limiting factors in conventional fluorescent lamps. The life of electrodeless fluorescent lamps is substantially higher than that of conventional fluorescent lamps and can reach 100,000 hours.
A high power (50–500 watts) electrodeless fluorescent lamp operated at a frequency of 25–1000 kHz is disclosed in U.S. application Ser. No. 10/964,372, filed Oct. 13, 2004. A bulbous lamp envelope with a reentrant cavity is fabricated of glass and is filled with an inert gas (argon, krypton or xenon) and mercury vapor. An inductively coupled discharge is ignited and maintained in the lamp envelope by an azimuthal electric field induced in the envelope by a magnetic field. The magnetic field is generated by a high frequency current in an induction coil wrapped around a ferrite core which is positioned in the reentrant cavity.
An exhaust tubulation is sealed to the reentrant cavity on the cavity axis. A mercury amalgam is held in the tubulation by several glass pieces. The position of the amalgam is selected to keep the mercury vapor pressure in the lamp envelope near 6 mTorr (milliTorr) when the lamp is operated within an ambient temperature range of −20° C. to +70° C.
To remove heat from the ferrite core so as to keep its temperature below the Curie point, a cooling structure is utilized. The cooling structure includes a cooling tube of high thermal conductivity metal or ceramic positioned inside the ferrite core, and a heat sink of a high thermal conductivity material located at the bottom of the lamp envelope. The cooling tube and the heat sink are thermally and electrically connected.
A dielectric spacer is positioned between the ferrite core and the inner wall of the reentrant cavity to create a gap between the cavity wall and the ferrite core of 3–5 mm. The gap decreases heat transfer from the cavity wall to the ferrite core and the coil wire. Such an arrangement maintains the temperature of the ferrite core and the induction coil wire below 200° C. at a lamp power up to 300 watts.
However, when the lamp is operated in a base down position and an ambient temperature of −20° C. and lower, the temperature of the bottom of the lamp envelope can be substantially lower than the temperature of the amalgam in the exhaust tubulation. As a result, the envelope bottom operates as the cold spot and thereby controls mercury pressure in the lamp envelope. This leads to a decrease of mercury pressure in the lamp envelope below 6 mTorr and results in a substantial decrease of lamp light output.
Accordingly, there is a need for improved electrodeless fluorescent lamps which have light output at low ambient temperatures that is comparable to the light output at room temperature.